It's called an "anacrusis" or pickup measure.
1/ Easiest way is to add one when you start a new score, just tick the checkbox and add the value:
2/ Another way, e.g. if you already have created a score without a pickup measure, is to modify the first measure, by right clicking on it, then selecting "measure properties". There you can change the actual duration to 1 for example:
As was commented below, you should also tick the "Exclude from measure count" checkbox, since (as explained here: Should pickup measures have measure numbers?) pickup measures aren't considered measure 1.